The invention relates to an analyzer for optically determining the concentration of a selected constituant of a fluid.
Optical fluid analyzers for both gas and liquid fluids are known. One, for example, is disclosed in the Assignee's U.S. Pat. No. 4,069,420. The analyzer disclosed in this patent uses the extent of infrared absorption at a wavelength at which a selected constituant of the fluid to be analyzed has a characteristic absorption to indicate the concentration of the constituant.
A rotating chopper disc alternately transmits the infrared light along separate optical paths from a single, infrared source through separate chambers to a single detector, one of the chambers containing the fluid to be analyzed and the other containing a reference specimen of known absorption. Electrical signals from the single detector are processed in synchronism with the path alternation to provide alternate signals corresponding to the absorption in the sample and reference chambers. The alternate signals are then integrated (subtracted) to provide a difference signal which indicates the concentration of the selected constituant in the fluid. Providing the separate optical paths to obtain the separate sample and reference signals for integration tends to cancel the effect fo certain variations in the operation of the analyzer such as variations in the infrared source for more accurate analyses.
In addition, each interval between the alternate, chopped optical signals is used to clamp or re-zero the electrical signal processing and each signal from the reference chamber where the absorption is known is used to restandardize the gain of the electrical signal processing. These three features, the separate optical paths, the zero clamp and the reference gain control, thus each contribute to making the analyzer described in U.S. Pat. No. 4,069,420 highly accurate.
Further refinements to the analyzer disclosed in the beforementioned patent are, however, disclosed in the Assignee's further U.S. Pat. No. 4,153,837. These further refinements relate to barometric pressure compensation, temperature compensation, and calibration display.
Neither of the analyzers disclosed in the patents, however, has an arrangement to minimize the effect of background absorption. The background absorption is the overall or base level of optical absorption of the fluid to be analyzed from which the absorption of the selected constituant departs at its characteristic wavelength to an extent indicating the concentration of the selected constituant. When the level of background absorption is high in relation to the characteristic absorption of the selected constituant, as it is for many liquids because of their turbidity and other properties, accurate measurement of the level of the small change in the absorption at the characteristic wavelength of the constituant selected to be analyzed for determining the constituant concentration becomes very difficult even for high concentrations of the constituant, and more difficult still for low concentrations.
An analyzer which could reduce the background level of the signal to be analyzed could thus be even more accurate than the patented analyzers and particularly useful for analyzing liquids.
Highly accurate analyzers for liquid, particularly, also present a further problem. Because the fluid analyzer must produce a signal the actual or absolute value of which indicates the concentration rather than merely accurate relative signals, aging, temperature or other internal changes in the signal-processing components will affect the analyzer operation. The analyzer therefore has to be periodically reset for accuracy using a standard fluid of known constituant concentration. Pure gases for such standard fluids in gas fluid analyzsis are reasonably available, but standard liquid fluids often require special, difficult preparation and handling and are more difficult to purge from the analyses after use to present greater risk of contaminating subsequent analyzes. An analyzer which could be adjusted for internal changes without using a standard fluid and thus less frequently restandardized with a standard fluid would thus be particularly useful for analyzing liquids.